The probe was launched to look into Russian election interference and if Trump’s campaign coordinated with the Russians

Law enforcement officials in the US reportedly became so concerned about President Donald Trump’s behaviour after he sacked FBI Director James Comey that they began investigating whether he had been working for Russia.

A report from the New York Times cites unnamed former law enforcement officials and others familiar with the investigation.

Trump was reportedly assessed to determine whether he was a threat to national security.

The inquiry forced counterintelligence investigators to evaluate if Mr Trump was deliberately working for Russia or if he had unintentionally been influenced by Moscow, according to the report.

The NY Times suggests that FBI agents and some top officials grew increasingly suspicious of Mr Trump’s ties to Russia during the 2016 presidential campaign.

Though it is claimed they did not launch an investigation at that time because they were not sure how to approach such a sensitive and vital probe, according to the sources.

President Trump (Getty Images)

But Mr Trump’s behaviour in the days around Mr Comey’s May 2017 firing, specifically two instances in which he seemed to tie Mr Comey’s ousting to the Russia investigation, helped trigger the counterintelligence part of the investigation, according to the Times’ sources.

Robert Mueller took over the investigation when he was appointed special counsel soon after Mr Comey’s firing.

The overall investigation is looking into Russian election interference and whether Mr Trump’s campaign coordinated with the Russians.

The Times says it is unclear whether Mr Mueller is still pursuing the counterintelligence angle.

Mr Trump’s lawyer Rudy Giuliani told the Times that he had no knowledge of the inquiry but said that since it was opened a year and a half ago and they had not heard anything, apparently “they found nothing”.

Mr Trump has repeatedly denied collusion with Vladimir Putin and the Kremlin.